Regulatory mechanisms of ryanodine receptor/Ca2+ release channel revealed by recent advancements in structural studies

Ryanodine receptors (RyRs) are huge homotetrameric Ca²⁺ release channels localized to the sarcoplasmic reticulum. RyRs are responsible for the release of Ca²⁺ from the SR during excitation–contraction coupling in striated muscle cells. Recent revolutionary advancements in cryo-electron microscopy have provided a number of near-atomic structures of RyRs, which have enabled us to better understand the architecture of RyRs. Thus, we are now in a new era understanding the gating, regulatory and disease-causing mechanisms of RyRs. Here we review recent advances in the elucidation of the structures of RyRs, especially RyR1 in skeletal muscle, and their mechanisms of regulation by small molecules, associated proteins and disease-causing mutations.

Deletion of IP3R1 by Pdgfrb-Cre in mice results in intestinal pseudo-obstruction and lethality

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are a family of intracellular Ca²⁺ release channels located on the membrane of endoplasmic reticulum, which have been shown to play critical roles in various cellular and physiological functions. However, their function in regulating gastrointestinal (GI) tract motility in vivo remains unknown. Here, we investigated the physiological function of IP₃R1 in the GI tract using genetically engineered mouse models.

METHODS

Pdgfrb-Cre mice were bred with homozygous Itpr1 floxed (Itpr1^f/f) mice to generate conditional IP₃R1 knockout (pcR1KO) mice. Cell lineage tracing was used to determine where Pdgfrb-Cre-mediated gene deletion occurred in the GI tract. Isometric tension recording was used to measure the effects of IP₃R1 deletion on muscle contraction.

RESULTS

In the mouse GI tract, Itpr1 gene deletion by Pdgfrb-Cre occurred in smooth muscle cells, enteric neurons, and interstitial cells of Cajal. pcR1KO mice developed impaired GI motility, with prolonged whole-gut transit time and abdominal distention. pcR1KO mice also exhibited lethality as early as 8 weeks of age and 50% of pcR1KO mice were dead by 40 weeks after birth. The frequency of spontaneous contractions in colonic circular muscles was dramatically decreased and the amplitude of spontaneous contractions was increased in pcR1KO mice. Deletion of IP₃R1 in the GI tract also reduced the contractile response to the muscarinic agonist, carbachol, as well as to electrical field stimulation. However, KCl-induced contraction and expression of smooth muscle-specific contractile genes were not significantly altered in pcR1KO mice.

CONCLUSIONS

Here, we provided a novel mouse model for impaired GI motility and demonstrated that IP₃R1 plays a critical role in regulating physiological function of GI tract in vivo.

Ca2+-mediated activation of the skeletal-muscle ryanodine receptor ion channel

Cryo-electron micrograph studies recently have identified a Ca²⁺-binding site in the 2,200-kDa ryanodine receptor ion channel (RyR1) in skeletal muscle. To clarify the role of this site in regulating RyR1 activity, here we applied mutational, electrophysiological, and computational methods. Three amino acid residues that interact directly with Ca²⁺ were replaced, and these RyR1 variants were expressed in HEK293 cells. Single-site RyR1-E3893Q, -E3893V, -E3967Q, -E3967V, and -T5001A variants and double-site RyR1-E3893Q/E3967Q and -E3893V/E3967V variants displayed cellular Ca²⁺ release in response to caffeine, which indicated that they retained functionality as caffeine-sensitive, Ca²⁺-conducting channels in the HEK293 cell system. Using [³H]ryanodine binding and single-channel measurements of membrane isolates, we found that single- and double-site RyR1-E3893 and -E3967 variants are not activated by Ca²⁺ We also noted that RyR1-E3893Q/E3967Q and -E3893V/E3967V variants maintain caffeine- and ATP-induced activation and that RyR1-E3893Q/E3967Q is inhibited by Mg²⁺ and elevated Ca²⁺ RyR1-T5001A exhibited decreased Ca²⁺ sensitivity compared with WT-RyR1 in single-channel measurements. Computational methods suggested that electrostatic interactions between Ca²⁺ and negatively charged glutamate residues have a critical role in transducing the functional effects of Ca²⁺ on RyR1. We conclude that the removal of negative charges in the recently identified RyR1 Ca²⁺-binding site impairs RyR1 activation by physiological Ca²⁺ concentrations and results in loss of binding to Ca²⁺ or reduced Ca²⁺ affinity of the binding site.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

PI(3,5)P(2) controls membrane trafficking by direct activation of mucolipin Ca(2+) release channels in the endolysosome

Membrane fusion and fission events in intracellular trafficking are controlled by both intraluminal Ca(2+) release and phosphoinositide (PIP) signalling. However, the molecular identities of the Ca(2+) release channels and the target proteins of PIPs are elusive. In this paper, by direct patch-clamping of the endolysosomal membrane, we report that PI(3,5)P(2), an endolysosome-specific PIP, binds and activates endolysosome-localized mucolipin transient receptor potential (TRPML) channels with specificity and potency. Both PI(3,5)P(2)-deficient cells and cells that lack TRPML1 exhibited enlarged endolysosomes/vacuoles and trafficking defects in the late endocytic pathway. We find that the enlarged vacuole phenotype observed in PI(3,5)P(2)-deficient mouse fibroblasts is suppressed by overexpression of TRPML1. Notably, this PI(3,5)P(2)-dependent regulation of TRPML1 is evolutionarily conserved. In budding yeast, hyperosmotic stress induces Ca(2+) release from the vacuole. In this study, we show that this release requires both PI(3,5)P(2) production and a yeast functional TRPML homologue. We propose that TRPMLs regulate membrane trafficking by transducing information regarding PI(3,5)P(2) levels into changes in juxtaorganellar Ca(2+), thereby triggering membrane fusion/fission events.